1. Field of the Invention
The invention is related to the field of luminescence, and in particular, to phase shift measurement between an excitation light wave and a luminescent light wave.
2. Statement of the Problem
The concentration of oxygen in water is measured with a probe. The oxygen in the water interacts with a luminescent material on the outside of the probe. This interaction between the oxygen and the luminescent material results in a phenomenon known as luminescent quenching. Thus, the amount of luminescent quenching indicates the concentration of oxygen in the water.
In operation, the probe directs excitation light of one wavelength to the luminescent material. The excitation light causes the luminescent material to generate luminescent light at a different wavelength. Luminescence quenching affects the amount of time that the luminescent material continues to luminescence light. Thus, if the excitation light signal's intensity varies sinusoidally, the luminescence quenching affects the phase shift between the excitation light and the luminescent light. The probe measures the phase shift between the excitation light and the luminescent light to assess the amount of luminescent quenching. As a result, the probe processes the phase shift to determine the concentration of oxygen in the water.
Existing technology probes use a servo feedback loop to measure the phase shift between the excitation light and the luminescent light. In some cases, the servo feedback loop introduces additional phase shift until the excitation light and the luminescent light are in phase, and the amount of additional phase shift should match the phase shift between the excitation light and the luminescent light. In other cases, the servo feedback loop introduces additional phase shift until the servo system detects that the excitation light and the luminescent light are 90 degrees out of phase. The additional phase shift is subtracted from 90 degrees to obtain the phase shift between the excitation light and the luminescent light.
The luminescent material must remain exposed to the excitation light until the servo system settles, and the servo settling time may take several seconds. Unfortunately, exposure of the luminescent material to the excitation light may degrade the luminescent material. In addition, the components of the probe introduce unwanted phase shift that is incorrectly attributed to the oxygen in the water. This unwanted phase shift adds error to the oxygen concentrations that are determined by the probe.